1. Field of the Invention
This invention relates to a golf ball which is durable against repetitive hits.
2. Prior Art
From the past, it has been desired to improve the aerodynamics of golf balls. Various attempts have been made to increase the flight distance of golf balls, for example, by optimizing the shape and size of dimples to reduce a coefficient of drag and increase a coefficient of lift of the ball in flight.
Golf balls are generally classified into two-piece golf balls and wound golf balls. In the recent years, two-piece golf balls have become more popular partly because they follow a relatively straight trajectory and cover a longer distance as compared with wound golf balls. In fact, the market share of two-piece golf balls is outstandingly increasing.
Wound golf balls have the advantages of better hitting feel and control than two-piece golf balls. For example, wound golf balls using balata rubber as a cover stock are susceptible to spin in approach shots so that the balls can be stopped on the green as desired.
Ionomer resins are used as a base of the cover of two-piece golf balls and some wound golf balls. A golf ball using ionomer resin in the cover has a higher hardness than a wound balata ball. As compared with the wound golf ball, this golf ball receives a less spin rate upon approach shots so that it may be rather difficult to stop the ball on the green. Then, for the purpose of increasing a spin rate upon approach shots, several attempts have been made to improve the cover by tailoring the ionomer resin. None of ionomer resin covers proposed thus far are comparable to the balata rubber cover.
Another means for improving spin is to improve the club. For example, it has been proposed to increase a spin rate by improving the shape of furrows in the face of a short iron. When a ball is hit with such an improved short iron, despite an increased spin rate, there arises a problem that the dimples on the ball surface are collapsed, deformed or damaged by the club. The ball will shortly lose spin susceptibility as the ball is repeatedly hit.
The inventors carried out a fixed point hitting test on a commercially available two-piece golf ball A having a low hardness ionomer resin cover, a commercially available two-piece golf ball B having a high hardness ionomer resin cover, and a dimple-free ball C having a high hardness ionomer resin cover. The test used a sand wedge having a loft angle of 57.degree., the ball was hit 10 times at the same point at a head speed of 25 m/s, and a spin rate was measured on every hit. The dimple-free ball C showed little lowering of spin rate whereas dimpled balls A and B experienced a substantial lowering of spin rate after repeated hits. It was thus acknowledged that the spin performance decline mainly by deterioration of the shape of the dimples.
Therefore, an object of the invention is to provide a novel and improved golf ball wherein the configuration of dimples is improved to prevent the spin performance from lowering by repetitive hits without detracting from the dimples' own effect of increasing flight distance, that is, having improved flight performance stability and durability.